Fellwar Stone

{T}: Add one mana of any color that a land an opponent controls could produce.

Firemind Vessel

Firemind Vessel enters tapped.
{T}: Add two mana of different colors.

Tome of the Guildpact

Whenever you cast a multicolored spell, draw a card.
{T}: Add one mana of any color.

Chromatic Lantern

Lands you control have "{T}: Add one mana of any color."
{T}: Add one mana of any color.

Niv-Mizzet, Supreme

Flying, hexproof from monocolored
Each instant and sorcery card in your graveyard that's exactly two colors has jump-start. (You may cast that card from your graveyard by discarding a card in addition to paying its other costs. Then exile it.)

Arcane Signet

{T}: Add one mana of any color in your commander's color identity.